This invention relates generally to a method to effect saccharification of cellulosic material, and more particularly to a method for the production of ethyl alcohol or ethanol from cellulosic waste material saccharified by an anhydrous liquefied hydrogen halide such as hydrogen chloride.
The hydrolysis of cellulose yields a mixture of simple reducing sugars, mainly glucose. These hydrolysis products are convertible by fermentation to ethyl alcohol which can be used as a liquid fuel to replace gasoline. In terms of available energy, expressed either as the heat of combustion of cellulose or of the glucose or alcohol theoretically obtainable therefrom, a pound of cellulose is equivalent to 0.35 lb. of gasoline.
Billions of tons of carbon are fixed every year on the land area of the earth by photosynthesis, out of which about half appears in the form of cellulose. The sheer magnitude of this potential source and the fact that petroleum-derived fuels are becoming increasingly scarce and more expensive, has awakened great interest in the exploitation of otherwise unused waste cellulosic materials.
About two hundred ninety million tons per year of residual, institutional and commercial solid wastes containing approximately fifty percent paper and other cellulosic materials are generated in the United States. Moreover, about sixty million tons of bagasse are available. These altogether contain about one hundred and twenty million tons of usable cellulose, the energy equivalence of which is nearly a fifth of the current U.S. gasoline consumption. Furthermore, 2.3 billion tons per year of agricultural wastes in the United States having a high cellulose content, increase this supply significantly. Despite these many and varied sources of available cellulose, a major industrial alcohol fermentation process can only be successful if the reducing sugars can be derived from a cellulosic material at a sufficiently low price.
Early efforts in this direction were mainly geared to the acid hydrolysis of wood products to sugars. In recent years, various methods have been proposed to carry out cellulose degradation through enzymatic means. Thus U.S. Pat. Nos. 3,642,580; 3,764,475 and 4,009,075 effect the conversion of cellulose to simple sugars by enzymatic hydrolysis, the resultant sugars then being fermented by years to produce alcohol.
Hydrolysis is a chemical reaction in which water reacts with a compound to cause the decomposition or splitting thereof. Water in the form of its hydrogen and hydroxyl ions adds to the cleaved compound. Hydrolysis is generally catalyzed by ions. In the absence of ions, hydrolysis may be a very slow process. Thus the use of an acid as a catalyst increases the concentration of hydrogen or hydroxyl ions with a corresponding rise in the rate of hydrolysis. The hydrolysis of some organic compounds is also catalyzed by certain enzymes.
It is known that the polysaccharide components of wood and other cellulosic materials can be partially or completely hydrolyzed by acid catalyzed reactions in which the cellulose is converted to glucose and the hemicellulose mostly to xylose. The resultant syrupy mixture or molasses of wood sugar may be used as cattle feed, or it may be fermented to alcohol. Alternatively, the glucose derived in this manner may be separated from the wood sugars and purified.
Cellulose is a D-glucose polymer with .beta. (1-4) linkages. Related compounds are polymers of D-xylose with .beta. (1-4)linkages and side chains of arabinose and other sugars. The glycosidic bonds between these various sugar sub-units --0-- are akin to ether bonds, acetal bonds and hemiacetal bonds, which bonds are hydrolyzable by acids. An aqueous acid not only acts as a catalyst for these reactions, but also supplies water thereto. This characteristic of an aqueous acid gives rise to a major drawback in the hydrolysis of cellulose.
When, for example, cellulosic material is treated with an aqueous solution of hydrogen chloride (hydrochloric acid) as a catalyst promoting hydrolysis, those cellulose molecules having exposed glucose sub-units will be hydrolyzed. But the free glucose resulting from this process is hydroscopic in nature and renders the remaining cellulose inaccessible to chemical decomposition. The same problem arises with the enzymatic hydrolysis of cellulose.
As a consequence, existing processes using aqueous acids or enzymes to catalyze the decomposition of cellulosic compounds by hydrolysis give a poor yield or are very energy consumptive. This renders such processes uneconomic and impractical when, for example, the controlling consideration is the comparative cost of conventional fuels and an equivalent amount in terms of energy content of ethyl alcohol produced by saccharification and fermentation.